Add Saxon.XML()

* Add Nokogiri.XML()-alike method
* extract common S9 API into Saxon::S9API file
* Extract the to_inputstream / to_stream_source methods from XSLT to a
helper module for use in Saxon::XSLT and Saxon::XML
* Extract spec fixture path helper into helper module and include via
spec_helper.rb
Refactor to use shared Saxon Processor instance
Only allow XSLT transform to accept Document object as input